Modulation of cellular signaling mechanisms by qualitative differences in dietary proteins and their metabolites were studied at the level of: a) Effect of pyrroline 5-carboxylate (P5C) as a signaling and regulatory molecule b) P5C as a diet-dependent scavenger of nitric oxide. c) Enzymes mediating the formation and degradation of P5C, d) Imidodipeptides. a. Pyrroline 5-carboxylate as an effector molecule. P5C stimulates PRPP and purine ribonucleotide synthesis synergistically with platelet-derived growth factor. It also increases the incorporation of thymidine in serum-activated cells. Recent studies have shown that P5C is synergistic with hypoxia in inducing iNOS. Since NO can serve as an ubiquitous signaling molecule, some of the previously observed P5C effects may be mediated by NO. b. Pyrroline 5-carboxylate as a diet-dependent scavenger of nitric oxide. P5C is a circulating metabolite and its levels in plasma undergo diet-dependent diurnal fluctuations. We recently found that P5C interacts with nitric oxide to form nitrosoproline. The scavenging of NO by P5C readily occurs at physiologic pH and at concentrations of P5C found in plasma. Biologic regulation of this process is being intensively studied. c. Enzymes mediating the synthesis and degradation of pyrroline 5-carboxylate. Because P5C has the aforementioned regulatory effects, the synthesis and degradation of P5C is of interest. Specifically, ornithine aminotransferase, P5C reductrase and P5C dehydrogenase are being studied. Using Northern blots, we are studying the expression of these enzymes in animals and in cultured cells. We found that the regulation of hepatic ornithine aminotrasferase by diet requires the presence of p53. In transgenic p53 knockout mice, the regulation by carbohydrate is abrogated. d. Imidodipeptides. Dipeptides containing proline or hydroxyproline originate from either tissue matrix degradation or from protein nutrition. They circulate in plasma and are delivered to tissues where they are hydrolyzed by prolidase. Thus, prolidase is a potential interface between protein nutrition and matrix breakdown. Our studies showed that the level of cellular prolidase is regulated by extracellular collagen acting through integrin receptors. Thus, the hydrolysis of imidodipeptides, the final degradative products of matrix collagen, is responsive to cellular interaction with extracellular matrix. We are studying the regulation of this enzyme on the molecular level.